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Do a Compression Test
As published in "Your Classic" Dec 1991

screw-in type
compression tester

A compression test will quickly tell you what state your engine is in,
and what is needed to restore it back to health.

Here we explain how to diagnose the problems.

An engine compression check will quickly tell you whether pistons, rings or bore, are badly worn or damaged. It'll tell you if the inlet and exhaust valves are burned or failing to seat properly. It can even tell you if the cylinder head gasket has failed.

It is very simple to carry out too. All you need is a willing assistant, a purpose-made and fairly cheaply available compression tester, a suitable socket spanner and wrench to remove the spark plugs, a small trigger-type can of engine oil and a pen and paper.

Begin by warming the motor to its normal operating temperature. Remove all the spark plugs, disable the ignition system by disconnecting the coil connection, then attach the compression tester to each plug hole in turn, while an assistant cranks the engine on the starter and you note the readings.

Simple, but there are important points to consider. First you'll have to zero the gauge after you've carried out each test and written down the result. Secondly, make sure the gauge, if it's of the cheaper push-in rather than screw-in variety, seats properly against the cylinder head to avoid falsely pessimistic reading.

And finally you must make sure that not only does the engine turn over at or near its normal cranking speed for long enough for the reading on the gauge to stabilise, but that your assistant keeps the throttle wide open throughout. The high manifold depression created by a closed throttle would provide a falsely low reading.

Normal readings vary widely depending on the engine and its compression ratio, of course, but somewhere in the region of 175psi is typical. And in all cases it's the consistency of the figures which is as important as their absolute value. Consistently low readings on all cylinders indicate general wear and tear which probably won't cause the engine too much of a problem; low readings in one or more cylinders (not necessarily adjacent) suggests that they contain a specific fault.

Let's assume, for example, that on a typical four-cylinder engine you've obtained readings of 150psi, 140psi, 50psi and 120psi. So what do your figures all mean? Well, basically that the engine probably runs very badly indeed! It'll be noticeably down on overall power, with a very poor throttle response, and if it runs on all four cylinders at higher speeds, the chances are that at idle it will run on only a couple.

The next step is to try to find out why. Remember there are essentially only three possible sources of the problem: rings and/or bores, valves and head gasket. Inject a small quantity of oil into each cylinder, let it find its way around the circumference of the pistons and down onto the rings for a minute or two then repeat your tests with the gauge. (Drape a cloth over the other plug holes so you don't get covered in oil.)

It's unlikely that you'll see any really dramatic improvement on the cylinder which could manage only 50psi (that points fairly conclusively to a valve with a very badly eroded seat or a broken piston ring) but if the 120psi cylinder now manages 145psi, then you've probably got wear of the rings and/or bores. If on the other hand, the 120psi reading stays resolutely the same even with the extra sealing provided by the oil, then it's fairly safe to assume that it's the valves that are at fault (particularly in view of that 50psi reading in an adjacent cylinder).

On the pushrod-operated overhead-valve engines with easily adjustable valve clearances you can try repeating the test with the adjusters slackened right off to make sure the valves aren't being held off their seats, but the chances are that the seats are pitted or the valves themselves actually burned in which case there's no alternative but to whip the head off for inspection.

So how can you tell if the head gasket has blown? Well, a compression check alone is no certain proof but, it can nonetheless give you a fairly reliable indication. Suppose that two adjacent cylinders are well down or compression at, say, 100psi, and that you are reasonably certain the valves and bores are in good condition. That points fairly conclusively to a gasket failure between the two offending cylinders; and likewise a low reading in one cylinder can often confirm one's suspicions that a leak of combustion gases into the cooling system is causing a consequent build-up of pressure and eventual loss of coolant. (Check the cooling system with a purpose-made pressure tester.)

It is also possible, if unlikely, that you'll encounter excessively high readings. These may be caused by excessively wide valve clearances (check with a feeler gauge) or even by the presence of fluid (oil, petrol or water) in the cylinder. The oil could have been sucked in past faulty rings or valve-guide oil seals, the petrol could indicate a serious problem in the carburettor, and the water could well be leaking in past a faulty head gasket or even through a crack in the cylinder wall.

In all these cases the only way you'll ever confirm totally the compression tester's findings though, is by removing the cylinder head completely. 1999 - 2021